The invention relates to an achromatic microscope objective having a magnification within the range of 35 to 45 times, and a numerical aperture of more than 0.60, the objective consisting of three components of which the first is a positive component and the third is a cemented doublet formed from a negative element and a biconvex element.
More than fifteen years ago, a microscope objective known as a Planachromat 10/0.22 was on the market. This consisted of three lens components, with a collecting first component and second component, and a third cemented component. The advantage of a construction with only four lenses, obtained in the case of this objective having a small magnification of 10.times. and the relatively small aperture of 0.22 could not, until now, be transferred to a microscope objective of larger aperture and higher magnification and therefore short focal length.
There have been many attempts to construct achromatic microscope objectives of as few lens elements as possible. Objectives consisting of five elements and sometimes of six elements are known, but such objectives are relatively expensive. For example, German Pat. No. 2,424,399 discloses a microscope objective having a magnification of 40.times. and a numerical aperture of 0.65, and this consists of three components having a total of five elements. In this construction, the first component is a collecting individual element, while both the second and third components are cemented components having a negative and a biconvex collecting element. In this known objective, the refractive power required for the formation of a real image is distributed over three collecting lenses. In order to compensate for the spherical aberration and the negative chromatic axial aberration produced by them, two diverging lenses are provided.
This known objective is relatively expensive, as a result of the use of a total of five lens elements. Furthermore it requires a relatively high expenditure of manufacturing technique in order to produce the two cemented components.
The object of the present invention is to provide a relatively low cost achromatic microscope objective having a magnification within the range of 35 to 45.times., and a numerical aperture greater than 0.60, so designed that good correction of all image errors is provided with the use of only low cost mass produced glasses, and which can be built with only relatively low manufacturing expenditure.
The invention starts from an objective in which the first and second components are positive single elements and the third component is a cemented doublet made up of a negative element and a biconvex collecting lens, and it is characterized by the proper choice of the glasses used in the respective elements. The second component is developed as a collecting single element with a refractive index n.sub.d within the range of 1.55 to 1.70 and an Abbe number v.sub.d within the range of 45 to 68. By way of example, glass obtainable under the designation SK 16 can be used for this lens. Glass of this designation is shown in the glass catalog of the glass manufacturing firm of Jaener Glasswerk Schott & Genossen, of Mainz, Germany.
This development makes it possible to keep the spherical aberration and the negative chromatic axial aberration caused by the total of three collecting lenses so slight that dependable compensation is possible by using merely one diverging lens. The diverging lens which is present in the third component is made of glass having an index of refraction n.sub.d within the range of 1.65 to 1.97 and an Abbe number v.sub.d within the range of 20 to 40, that is to say, made of material having a high refractive index and low Abbe number. For this, glass available under the designation SF 56 in the above mentioned glass catalog can be used, for example.
The biconvex positive lens cemented to this diverging lens to form the doublet is made of material having a low refractive index, within the range of 1.46 to 1.57, and a relatively high Abbe number within the range of 50 to 85. For this element, glass obtainable under the designation K7 in the above mentioned glass catalog can be used, for example.
The front or first component is made of material having a refractive index n.sub.d within the range of 1.55 to 1.70 and an Abbe number v.sub.d within the range of 45 to 68. Glass available under the designation SK 2 in the above mentioned glass catalog can be used, for example. Preferably this front lens element is of concave-convex curvature and has a center thickness which can still be easily produced from a manufacturing standpoint.
In order to obtain the desired chromatic magnification difference, the air space between the second single component and the third lens component is so selected, according to the invention, that it is less than 0.8 F, where F is the equivalent focal length of the objective.
The microscope objective of the present invention is characterized particularly by the fact that, with full utilization of the selection of low priced mass produced glasses possible from glass catalogs, a dependable correction of image defects is obtained with the use of only four lens elements, while the cost of manufacture is maintained relatively low.